The present invention relates to a clock generating apparatus and a clock generating method suited to STC control necessary to an MPEG-2 TS System.
The MPEG-2 System defines a system for multiplexing individual streams such as an arbitrary number of encoded video, voice and additional data and reproducing them with each of them in synchronization. The MPEG-2 System defines two kinds of system in order to support a wide range of applications.
One is a program stream (PS), where one program is configured in one stream. The program stream is intended to be applied to transmission and accumulation of data in an environment where no error occurs. As the program stream can reduce redundancy, it is used for a digital storage medium using a strong error-correcting code, such as a DVD.
The other is a transport stream (TS), where multiple programs can be configured in one stream so that it can support digital broadcasting and the like. The transport stream is intended to be applied to an environment where a data transmission error occurs such as broadcasting or communication network. It has greater redundancy than the program stream.
The MPEG-2 TS System has MPEG-2 encoded video signals, voice signals, additional data, control signals and the like stored in transport stream packets (hereinafter referred to as TS packets) of 188 bytes.
In the MPEG-2 TS System, a transmitter (encoder) transmits a TS packet including program clock reference (hereinafter referred to as PCR) at regular time intervals in order to eliminate a difference in operation speed between the transmitter and a receiver (decoder) due to mutual clock deviation. The PCR is a value having a predetermined bit number (unit of 42 bit: 27 MHz).
Conventionally, the receiver compares the PCR included in a received TS packet with time information (a value counted on the receiver side, which is a count value assuming 27 MHz in advance on initialization) of a system time clock (hereinafter referred to as STC) for decoding an MPEG-2 encoded signal so as to increase or decrease voltage of a voltage controlled crystal oscillator (VCXO) used to generate the STC. To be more specific, the receiver adjusts and synchronizes the STC based on the PCR (refer to Patent Document 1 for instance).
Furthermore, an audio master clock in synchronization with the STC is generated by using the STC outputted from the VCXO so as to perform decode processing of audio (refer to Patent Document 2 for instance).
However, there has been a problem that the VCXO which is essential in generating the STC is an outside part leading to increase in cost and increase in packaging area.
[Patent Document 1] Japanese Patent Laid-Open No. 2006-134390
[Patent Document 2] Japanese Patent Laid-Open No. 2003-87229